Recently, in the field of injection molding, high pressure injection molding units have become commercially available in which the molding pressures may be as high as 30,000 p.s.i., which is a significant increase from the molding pressures that have been previously used. The purpose of the higher molding pressures is to permit the molding of stronger, stiffer plastic materials in thinner wall thicknesses, for example in the manufacture of containers, in order to keep the unit price of the molded product as low as possible in the face of rising plastic raw material prices.
It has been found that with this increase in injection pressures, certain technical problems of operation have increased. Particularly under high molding pressures, it is possible for the injection mold core to laterally shift with respect to its mating injection mold cavity by a number of thousandths of an inch during the molding process. Particularly the situation where thin-walled containers are being manufactured, such core shift can significantly reduce the quality and uniformity of the molded products. The core shift can interfere with the production of thin-walled containers and other articles requiring a uniform wall thickness.
In accordance with this invention, a system is provided for assuring the proper positioning of a mold core in its mold cavity, even at high injection pressures, to permit the molding of thin-walled structures down to a wall thickness of 0.02 inch or the like, with a mold concentricity tolerance being maintained at a value on the order of 0.0002 inch. At the same time, the centering system of this invention can operate with significantly reduced wear, for long and reliable commercial manufacturing of plastic containers and other articles.
While hardened leader pins and hardened bushings for receiving the leader pins may be used in this invention, it has been found that this conventional expedient alone is not capable of holding good cavity to core concentricity under the high molding pressures that are now being used in the industry. Likewise, the conventional conical taper lock rings for providing engagement between the core and cavity can provide concentricity at pressures of 20,000 p.s.i. or less, but are less capable of providing good concentricity to the mold core and cavity at higher molding pressures. Additionally, the tapered surfaces of conventional taper locks between a mold core and cavity can cause a certain portion of the mold cavity expansion force created by high molding pressure to be resolved into a force vector that urges the core and cavity apart. Thus, any retraction motion that results can permit undesirable flashing to form on the molded object. By this invention, that problem is also reduced.